DC Furnace Arc Problems 2

[thedillweed]

I work in a structural steel mill which uses a DC Electric Arc Furnace (13.8 kV). We have been having problems for quite some time with the electrode arcing to the furnace roof and damaging the water cooled panels. I am currently researching ways that I can electrically isolate the roof from the body of the furnace. I have considered using an insulating ring made of muscovite or phlogopite which would be attached to the underside of the roof rim. However, I have doubts as to whether this material could withstand the heat (~2800F) for an extended period of time or if it will even serve as an effective electrical isolator. Has anyone else ever dealt with this problem, and if so can you give me any advice?

 

[NickelMet]

Who manufactured the furnace? Their engineers should be able to help you as a first run at the problem. If you don't like their answer or it doesn't work, you could consult with other furnace manufacturers.

Also, do you have any contacts in AISE/ISS? There may also be information available from their conference proceedings (especially in the EAF conferences).

~NiM

 

[NickelMet]

Just remembered one more source: your refractory vendor. You never know where information may pop up.

~NiM

 

[Acerero]

I might be able to help you. We have two dc-furnaces running without this problem. The solution is based on the type of anode you have. Please inform which type you have.

 

[thedillweed]

Hi Acerero. Our furnace has a single 28" diameter graphite top electrode. Also, our bottom electrode is the steel pin design with a conductive furnace bottom. What did you do to avoid the roof-arcing problem?

 

[Acerero]

The solution is a little bit complex to be explained but I will try.
Beginning with the power supply, it has to be "floating", the P and N bus should not be solidly grounded. They are grounded through a resistor array. The reason is to let either the P or the N bus to touch a grounded point without damage.
On the furnace, the anode has to be "isolated" from the shell. This is probably the most important issue. The anode will be connected to the P bus which is floating.
The shell and the roof are solidly grounded.
Having this configuration, you will avoid roof arcing.
You will probably have more doubts. If you can give me your e-mail, I can send you a couple of skethes to explain in detail.

 

[thedillweed]

Acerero,

I would definitely like more info. I'm not electrical, I'm mechanical, so some of the explanation may be a bit over my head. However, I can have one of our EEs look at it with me and try to help me understand it. My email is 'daphne@sphynxter.com' I really appreciate your help.

Daphne

 

[sr11]

Just recently stumbled across your DC EAF arcing question.
Did you succeed in fixing arcing problem on the roof? Our customer (DC EAF) not long ago has damaged a roof panel supplied by us just after 4 days of operation.
Appreciate your help.

SR

 

[Acerero]

We have been working with DC Furnaces for several years. Arcing will never be totally eliminated, however it can be reduced to a point where operation is reliable.
Do you have some findings on how the roof panel was damaged?

 

[sr11]

Acerero,
I didn’t see it myself, but they describe it as a pit hole – damage caused by the arc strike in the area closest to the electrode and possibly through the contact with the scrap. This problem is quite common here. It even reflected in the name of the panel: so called “sacrificial panel” or “protection ring”. Panel itself is quite narrow, only 8 pipes going around the delta. Roof is not solidly grounded, only through the contact with the shell.
Why do you recommend roof to be solidly grounded? Doesn’t it create electric potential difference between electrode and a roof (scrap – roof) and, inevitably, arcing? I don’t really understand how grounding the roof will prevent electrode-roof (scrap-roof) arcing. Furnace manufacturers have different views on roof grounding, judging by my observations of EAF in different plants, AC and DC.
SR

 

[marciogerep]

By this time you should have reliable results of possible attempts to solve your roof arcing problems. Could you please share your experience with me. I am working on the same problem now and your experience may be very valuable if you are willing to share it.